This invention relates to an optical data communication and location apparatus, system and method and transmitters and receivers for use therewith.
Communications systems heretofore have employed fixed band and spread spectrum radio frequency (RF) energy. However, radio based systems which have included portable transmitters suffer from serious drawbacks including their susceptibility to other RF noise sources.; overcrowding of RF channels; and, the unpredictability of areas where reception is interrupted by the construction materials used in the building. Further, the use of RF systems for locating mobile items or individuals through triangulation does not yield a practical system due to lack of resolution and the time delay in the many calculations required. RF locating systems are also not cost effective for use inside a building, owing to their complexity. Other portable location system utilized ultrasonics to transmit the data. Ultrasonic energy for data communication and locating systems have been found to be impractical because of echoes and data errors from ambient noise. The ultrasonic transducers used are generally fragile. Only low data rates are achievable because of relatively low ultrasonic bandwidth. Infrared systems, although portable, have awkward weight and sizes with limited battery life limiting widespread usage.
To permit the use of a battery of even the relatively large size and capacity, transmitted infrared power had to be held to a low value. However, to realize a reasonable signal-to-noise level at the receiver, line-of-sight signal paths over a controlled distance between the infrared transmitter and receiver were proposed. Such line-of-sight systems require aligning the transmitter and the receiver to establish and maintain the transmission path during the entire period of transmission. This was proposed to be accomplished by mounting a fixed receiver over a doorway to look vertically downward. The transmitter was to be worn in a pocket of a wearer positioned to emit infrared signals upward. For such a system to function, transmissions had to be frequent enough and of short enough duration so as to allow the receiver to detect a full transmitted message during the period of time that the transmitter moved through the doorway. A diffuse infrared system is shown in U.S. Pat. No. 5,062,151. While the system does not require line-of-sight transmission to achieve portability, the power consumption is so large as to require a multi-cell, rechargeable battery. A battery sized for portability must be recharged frequently, at least every other day. Aside from the need for many rechargers and the inconvenience, the requirement for recharging makes locating mobile inanimate objects such as equipment, files, etc. impractical because of the need to frequently retrieve the transmitter for recharging. The requirement that the transmitters used to locate personnel be periodically returned to a charger is undesirable, in that while they are charging for eight to 16 hours, they cannot perform their intended function. Also, if the wearer inadvertently forgets to recharge the transmitter, the transmitter cannot be used until it is recharged. The requirement for a multi-cell battery sets a lower limit on the size and weight of the portable transmitter making it more cumbersome to wear or more difficult to attach to small, mobile objects.
In the system disclosed in U.S. Pat. No. 5,062,151, the first and only notification that a battery charge is becoming depleted is that the person or object associated with the transmitter can no longer be located. To add a battery checker to detect a low battery without the battery checker itself adding significantly to the drain on the battery being monitored presents a problem.
It is known that transmitting data using infrared pulses in lieu of modulating an infrared carrier frequency can reduce dramatically the power consumption of the transmitter, and any reduction in power consumption translates into a smaller battery and a longer useful battery life. The transmitted data of previous infrared systems is comprised of packets of ones and zeros. The presence of an infrared pulse is interpreted as either a one or a zero. The absence of a pulse represents the opposite. Data words containing mostly ones (assuming ones are the presence of infrared pulses) consume vastly more power than those with mostly zeros. The data sent by each such transmitter has a different quantity of pulses. Therefore, the power consumption of each transmitter is different. This causes the battery recharge interval to be set at that required for the transmitter which transmits all ones.
The larger and more complicated the facility, the more the need for portable communication with, and the locating of personnel and mobile items. However, in previous systems when the quantity of transmitter codes is increased, the quantity of pulses required to define a transmitter increases, and therefore power consumption increases dramatically. Any additional data such as battery condition further adds to the current drain and resultant reduction in battery life. Further, the use of DC-to-DC converters to multiply the battery voltage to that necessary to drive infrared emitters in series wastes considerable power. Any DC-to-DC converter will have losses which reduce battery life. The obvious alternative of adding batteries in series to achieve the necessary voltage for high power transmissions suffers from a substantial weight and cost penalty. The use of a resistor in series with the emitters to control current consumes the battery power, increasing battery size and decreasing battery life.
Transmitters being portable are susceptible to being lost or damaged. It is therefore desirable to be able to easily reprogram replacements. Prior art transmitters had their identification codes programmed with hardware jumpers or switches. In large systems, the quantity of elements such as switches needed to hard code transmitter data is impractical and costly. While it was known that the identity codes could be stored in solid state non-volatile memory, such memory is costly and consumes significant power, adding to battery size and decreasing battery life. Another factor precluding the use of conventional memory is the complication of programming the identification code. The low currents and high circuit impedances needed for low power consumption and small battery size make the use of common, low cost, electrical contact material impractical. The infrequent use of contacts for programming causes thin layers of oxidation and contaminants to coat the contacts, making them unreliable.
In the communication system disclosed in U.S. Pat. No. 5,052,151 the room receiver wiring requirements were onerous for medium to large size systems. For cable runs of reasonable length, the wire gauge must be large due to the high current consumption of the receivers on the run. While the parallel address and data busses provide for the large data throughput required of such a communications system, the large quantity of conductors which must be connected at each room and at a central logic facility makes the cost of the installed system undesirably high.
In systems with multiple transmitters, there is a need to prevent successive collisions of transmitted infrared data from separate transmitters in the same location. An accurate time base is a requirement for asynchronous data transmission. An accurate, high speed clock is a requirement for low power infrared transmission. It is well known that quartz crystals, and in some applications, even ceramic resonators provide an excellent and stable time base for such communication. However, because of their very high stability, once the serial transmissions of two transmitters with stable clocks begin to collide, they will tend to continue to collide for a very long time. A significant limitation of previous prior art systems was that they either lacked a method to prevent successive collisions of transmitted infrared data due to two or more transmitters with synchronized transmit intervals, or lacked a method to detect corrupt data caused by a collision or they consumed additional power to prevent collisions. Such erroneous data causes the database to be corrupted. In U.S. Pat. No. 5,062,151 successive collisions were prevented by the use of a combination of two transmit intervals that are unique to each and every transmitter. However, this is cumbersome to program and consumes additional power during transmission. For systems with a large number of transmitters, there may not be sufficient quantities of unique transmit interval pairs to assign to each transmitter. There is therefore a need for a new and improved optical data communication and location system and transmitters and receivers for use in the same.